When cutting bores in tooth roots and in bone for location of inserted posts, pins, screws and implants in dental surgery and also in oral or orthopaedic surgery problems due to inaccuracy of the resulting diameter can arise especially where access is limited.
For secure location of the inserted pin or post it is necessary or at least desirable for the receiving bore to be cut with dimensions precisely matched to those of the pin, post or screw. However, in conditions of limited access, such as is the case where a bore is to be cut in a tooth root or jaw bone, it may be necessary for the bore to be cut with a hand drill or other manually controlled device whereby some degree of lateral movement and consequent unwanted enlargement of the bore may be inevitable.
More specifically, in the context of dental surgery an artificial crown can be fitted onto a sound tooth root by cementing a post into a bore in the root and then fixing the crown onto a projecting portion of the post.
Where the residual root and tooth structure extends above the gum this on its own or supported by the post can ensure long term, secure attachment between the post and the root. However, when the residual root structure terminates at or below the gum it can be more difficult to prevent loosening of the post in use.
With the aim of improving security of fixing it is well known to drill a bore in the root structure and to use a matching post which is cemented into the bore.
However, the diameter of the drilled bore tends to become enlarged over the diameter of the drill or reamer whereby a post having the same diameter as the drill bit tends to be too small in the bore i.e. fit badly and hence loosen in use. When a threaded bore and post have been applied this is particularly problematical since the post with attached crown may not be readily removable for re-cementing without breaking and removing the crown in so far as the crown has an irregular lower surface which mates with the top surface of the root structure thereby resisting unscrewing of the post.
Loosening of the post within the bore tends to occur more readily when the upper (coronal) regions of the root structure are over-reamed. This is common, and the fit tends to remain closer at the bottom (apical) end of the bore. This can encourage pivotal movement of the post under the action of transverse forces about a fulcrum (where it fits) and this can give rise to the generation of large, destructive forces to the root structure as well as the cementing medium.
It is known to use a post which is enlarged in its upper (coronal) end portion. This helps to avoid or limit damage by the fulcrum effect but it is difficult for a dentist using hand tools to form accurately the required shaped bore to receive and securely locate the special post profile.
A further problem in the cutting of bores is that heat is generated during the cutting operation and there is the problem of ensuring adequate removal or dissipation of this to prevent damage being caused to adjacent root material or tissue.